This invention relates to screen printing apparatus and, more specifically, to a device for controlling and adjusting the spring rate and leverage associated with the screen holding arms usually associated with such apparatus.
Printing machines in the screen printing industry typically print small cap images and larger T-shirt images. In most case, printers use wood or aluminum frames of various sizes, typically a 9".times.12" for caps and a 20".times.22" for T-shirts and the like. In the case of wood, typically a soft wood such as pine, which has the advantage of being light in weight, is employed in the frame construction, with individual frame sides generally about 1 inch thick and about 11/2 inches wide.
There are instances, however, where oversize frames are required for even larger images such as for pant legs, sleeves, decorative banners, etc., and in those cases where more than one image is to be provided on a single screen. In such cases, the oversize frames are significantly heavier than the largest of the normally used frames, i.e., the 20".times.22" frame. In fact, it is known to use 2.times.4's constructed from Kiln-dried hard wood in the construction of oversize frames. These larger frames have proven to be difficult and even unworkable, when used with conventional screen printing machines primarily due to their increased weight.
In a typical screen printing machine, a screen (or screen frame) holding arm is mounted for pivotal movement between a printing, or down, position to a non-printing, or up, position relative to a platen upon which rests the item to be printed. The screen holding arm is generally provided with a clamping or holding mechanism which grips a screen of the type described hereinabove. Most such machines employ extension springs extending between the machine head and a point intermediate the ends of the screen holding arm, to control movement of the screen and screen holding arm from the printing to the non-printing position and vice versa. Of course, these springs are designed to extend only to certain length, beyond which the spring is subject to permanent damage from plastic deformation. The springs currently in use in the screen printing industry are sufficient to handle the normal range of frame sizes up to the 20".times.22" size, and even slightly larger. However, for substantially larger and heavier frames, these springs are not sufficient as explained below.
In conventional screen printing machines, using conventional screen sizes, the inner point of attachment of the spring, i.e., at the machine head, is generally slightly below the pivot point of the screen holding arm. Typically, when the screen is raised to a non-printing position (generally about 60.degree. from horizontal) and released, it will remain in the raised position. At the same time, when the arm is lowered to a printing position, the arm will remain lowered, with the screen in contact with the item to be printed, so that the operator can use both hands to print the cap, T-shirt or other items.
When oversize frames are utilized in such machines, the weight of the frame overcomes the spring tension so that the frame will not stay in the up, or non-printing position. Thus, the operator must support the screen in the raised position to prevent it from falling to the lower, or printing position. This, of course, is an undesirable and even unworkable situation.
If larger, stiffer springs are used to accommodate these oversize frames, the overall flexibility of the machine is reduced because the operator cannot thereafter switch back to the smaller, lighter cap or T-shirt frames. This is because the larger, stiffer springs will keep the lighter frames in a normally biased upward position so that an operator would not be able to release the frame after lowering it into a printing position. It is, of course, important that the operator be able to free his hands to print the image when the frame is lowered. Thus, in the past, moving between extreme frame sizes thus involved a time consuming change of springs as well. In addition, the stiffer larger springs employed for oversize frames are typically made of hardened steel, and create a substantial risk of injury in the event of breakage.
Other approaches have been taken when utilizing the larger frames for controlling the movement of the screen holding arms. For example, compression rather than extension springs have been tried, as have pressurized compression gas cylinders. However, compression springs have usually not proven to be satisfactory and, in the case of gas cylinders, there typically is no ability to adjust the cylinders to change the rate at which the connecting (piston) rod extends or retracts within the cylinder.
Other manufacturers have employed turnbuckles attached to the end of the extension springs to regulate the tension, or extension, of the spring. It has also been attempted to employ four springs, rather than two, for each screen holding arm. Neither of these techniques has met with any significant degree of acceptance in the trade.
This invention broadly relates to improvements in screen printing machines in general, and in particular, to screen printing machines wherein extension springs, compression springs or gas compression cylinders are associated with screen holding arms, and may be adjusted simply and quickly to not only alter the spring rate, but to also provide a degree of leverage to facilitate movement of the screen holding arm between printing and non-printing positions and vice versa, so that the spring action can be made, in effect, uniform for virtually any frame. In other words, it has been discovered that by adjusting spring tension as well as the point of attachment of the springs or gas cylinder to the machine head relative to the plane of the pivot point of the screen holding arm, a conventional screen printing machine can accommodate normal as well as oversize screens without any necessity for time consuming spring changeover.
In addition, because of the adjustability provided by the invention, the hardened steel springs usually employed in prior art machines are not required. In fact, it is preferable that the coil springs in this invention be constructed of steel "piano" wire, which pose a considerably lesser degree of risk in the event of breakage.
Specifically, in one exemplary embodiment, the invention relates to a screen printing machines of the type comprising a printing head mounted on a base and carrying at least one screen holding arm. The arm is pivotally mounted to the head for movement about a horizontal axis such that the arm is movable downwardly toward an associated platen and upwardly away from the platen. There is also associated with each arm a pair of coil extension springs, arranged on either side of the arm, with first ends attached to the rotary head and second ends attached to the arm at a location intermediate the ends of the arm and spaced from the pivot axis.
A unique mechanism is provided in accordance with this exemplary embodiment for adjusting the coil extension springs in two respects. First, the mechanism allows the effective length of the spring to be altered, thereby changing the spring rate. Second, the mechanism effects vertical movement of the attachment point of the spring at the machine head above, below or within the horizontal plane passing through the pivot point of the screen holding arm, thereby changing the amount of leverage available to lift or lower the screen holding arm. In a single color printing machine, utilizing a single screen holding arm, each of the two associated extension springs would be provided with an adjustment mechanism as provided by this invention, while in a typical four color machine, four such mechanisms are employed to accommodate eight extension springs. It will be understood, of course, that the adjustment mechanism of this invention is equally applicable in machines with any number of screen holding arms.
Each extension spring is attached to the printing machine head adjacent the pivot pin for the screen holding arm via an adjustment connector which includes an eyelet formed at either end of an elongated throat portion. The connector throat portion extends horizontally between interior and exterior portions of the machine head. On the interior side, the inner eyelet is slidably received over a vertically extending, threaded bolt. The bolt itself is captured within said head for rotational movement with respect to the head, i.e., during rotation, the axial position of the bolt does not change.
The bolt threadably receives a pair of nuts, with the inner eyelet sandwiched therebetween and fixedly secured, as by brazing or the like, to each. The intermediate or throat portion of the connector, passes through a slot between two adjacent walls of the machine head. The outer eyelet of the connector receives a hook portion of an extension spring. In this way, rotation of the bolt causes the nut and the connector assembly to move upwardly or downwardly on the bolt, depending on the direction of rotation, and thus raise or lower the attachment point of the spring relative to the pivot pin.
It will be understood that a similar adjusting mechanism is provided with respect to the other extension spring on the other side of the screen holding arm.
In an alternative arrangement to this first exemplary embodiment of the invention, the spring adjustment technique as described above is applied to a four color machine where four screen holding arms are arranged at 90.degree. intervals about a rotatable machine head. In this case, the machine includes four fixed platens, also arranged at 90.degree. intervals about the machine head, enabling the screen holding arms to be rotated, successively, to the individual platens. In this embodiment, the spring adjustment mechanism is identical to that provided for the single color machine, but here, four of the adjustment bolts are provided, each accommodating two extension springs. Because of the arrangement of screen holding arms at 90.degree. intervals, it is convenient to have each bolt adjust one coil extension spring associated with each of two adjacent and mutually perpendicular screen holding arms as disclosed in greater detail hereinbelow.
As a result of this invention, the screen printing machine operator can adjust the spring rate and leverage available for lifting the screen holding arms to obtain the desired tension and control of the arm, regardless of the frame size. Thus, for the normal 9".times.12" or 20".times.22" frames, the operator would, through rotation of the two or more adjustment bolts, maintain the inner point of attachment of the extension springs just below the plane of the pivot point of the screen holding arm. In this configuration, the machine operates on the usual way so that, in the down or printing position, the operator may release the arm and therefore free his hands for printing the image. At the completion of the printing operation, the operator may then raise the screen holding arm to an up, or non-printing position and the arm will be held in the raised position by the extension springs.
When oversize frames are to be used, the operator simply rotates the adjustment bolts to raise the point of attachment of the spring into, or just above the plane of the pivot point of the screen holding arm. This not only elongates the spring to increase the spring rate, but also increases the leverage available for raising the arm. Now, even the larger, oversize frames will be maintained in the upper position when released, and the weight of the frame will cause the frame to remain lowered when in the printing position, allowing the operator to free his hands for printing the image.
It is to be understood that various settings for frames of different sizes and weights may be determined empirically and recorded in chart form, enabling the operator to repeatedly adjust the springs precisely according to frame size and weight, with the aid of scales or rules applied to the machine head adjacent the adjustable spring connectors.
In another exemplary embodiment of the invention, a compression gas cylinder is utilized to control movement of the screen holding arm. In this embodiment, the screen holding arm is also mounted for pivotal movement relative to the machine head. However, the pivot pin about which the arm rotates is mounted for vertical sliding movement. At the same time, a piston and cylinder unit is fixedly mounted at first and second pivot points on the screen holding arm and machine head, respectively. The second pivot point on the machine head is fixed, just beneath a pair of vertically oriented slots in which the pivot pin of the screen holding arm is slidably mounted. In this manner, the "spring rate" of the cylinder, as well as the available leverage, may be altered in much the same manner as the above described extension spring embodiment.
It will be further understood that this second exemplary embodiment is equally applicable to four-color, or four-head machines as well.
It should also be understood that compression springs fixed between the screen holding arm and head may be used in place of the compression gas cylinder, with the screen holding arm pivot pin adjustable as described above, to achieve the same advantages.
Thus, the present invention enables calibration, control and consistency with respect to movement of a screen holding arm carrying frames of various sizes and weights in a simple and accurate manner not previously available in the screen printing trade.
Other objects and advantages of the subject invention will become apparent with the detailed description of the invention which follows.